As information technology equipment and communications equipment, such as personal computers, video cameras and cellular mobile phones, rapidly come into widespread use in recent years, greater importance has been placed on the development of secondary batteries, such as lithium secondary batteries, serving as excellent power sources. In technical fields other than that of the information technology equipment and communications equipment, for example, in an automobile industry, too, high-output, high-capacity lithium secondary batteries for use in electric vehicles and hybrid vehicles as low-emission vehicles have been developed.
In the meantime, commercially available lithium secondary batteries that are currently on the market use an organic electrolytic solution or liquid electrolyte containing a flammable organic solvent as a solvent. Therefore, a safety device for restricting or avoiding a temperature rise in case of shortings need be mounted, or an improvement(s) in terms of the structure and/or materials for preventing shortings need be made.
On the other hand, an all solid-state lithium secondary battery, which is obtained by replacing the liquid electrolyte with a solid electrolyte and is formed entirely in a solid state, does not use a flammable organic solvent in the battery. Therefore, the all solid-state lithium secondary battery only requires a simple safety device, and is deemed excellent in terms of the manufacturing cost and the productivity.
The all solid-state lithium secondary battery includes a cathode layer, an anode layer, and an electrolyte disposed between the cathode layer and the anode layer, and the electrolyte consists of a solid. Accordingly, when only a cathode active material is used to form the cathode layer by powder molding, the electrolyte, which is a solid, is unlikely to penetrate into the cathode layer, and the interface between the cathode active material and the electrolyte is reduced, resulting in reduction of the battery performance. Thus, a cathode mixture containing a mixture of a powder of the cathode active material and a powder of the solid electrolyte is used to form a cathode layer, thereby to increase the area of the interface between the cathode active material and the electrolyte.
When the cathode layer is formed by powder molding, using the cathode mixture as described above, however, the interfacial resistance to movement of lithium ions at the interface between the cathode active material and the electrolyte (which may be simply called “interfacial resistance”) tends to increase. This may be because the cathode active material reacts with the solid electrolyte, to form high-resistance portions on a surface of the cathode active material, as described in a non-patent document titled “LiNbO3-coated LiCoO2 as cathode material for all solid-state lithium secondary batteries” by N. Ohta et al., Electrochemistry Communications (2007), vol. 19, p. 1486-1490. Accordingly, technologies for improving the performance of the all solid-state lithium secondary battery by reducing the interfacial resistance have been disclosed. For example, a cathode active material formed such that a surface of LiCoO2 (lithium cobalt oxide) is coated with LiNbO3 (lithium niobate) is disclosed in the above-identified non-patent document. By using the disclosed cathode active material, it may be possible to curb or prevent formation of high-resistance portions at the interface between the cathode active material and the solid electrolyte, and reduce the interfacial resistance.
However, when the surface of the cathode active material is coated with a coating layer, fine particles of the cathode active material may be mixed into the coating layer, and high-resistance portions may be formed at contact areas between the fine particles of the cathode active material included in the coating layer, and the solid electrolyte. In this case, the resistance between the cathode active material and the solid electrolyte cannot be reduced, resulting in increase in the resistance and reduction in the output of the resulting battery.